1. Field of the Invention
The present invention relates to a field emitter, and more specifically to a field emitter for emitting electrons from a sharpened projection.
2. Description of Related Art
The prior art field emitter will be described with reference to FIGS. 1A to 1C. FIG. 1A illustrates a field emitter used in an electron gun of an electron microscope. It has a structure having a sharpened needle 32 fixed to a filament 31 formed of a resistance wire of a refractory metal. This structure is disclosed in Japanese Patent Application Post-examination Publication No. JP-B-01-045695 (corresponding to U.S. Pat. No. 4,379,250) (called "Reference A" hereinafter) and Japanese Patent Application Pre-examination Publication No. JP-A-56-099941 (called "Reference B" hereinafter).
FIG. 1B shows the field emitter called a "micro field emitter". This micro field emitter is disclosed in, for example, C. A. Spindt et al's article carried in Journal of Applied Physics, Vol. 47, page 5248, 1976.
In FIG. 1B, the micro field emitter includes a resistor 35 formed through a contact part 34 on a conductive substrate 33, and a molybdenum projection 36 formed to be electrically connected to the resistor 35. This molybdenum projection 36 is accommodated in a cylindrical hole formed in an insulator film 37 and a gate metal film 38. In addition, the resistor 35 is surrounded by a separation layer 39.
An equivalent circuit of the field emitters shown in FIG. 1A and FIG. 1B is as shown in FIG. 1C. This equivalent circuit is constituted of a resistor A-B, a resistor B-C and a resistor B-D, and electrons are supplied from resistor ends A and C. Expressing this supplying of electrons in different words, this is a supplying of an electric current (having a negative sign). Electrons are emitted from a resistor end D into vacuum. Here, the signs A, B, C and D are used in common to FIGS. 1A to 1C.
It was reported in the above referred References A and B and Japanese Patent Application Pre-examination Publication No. JP-A-08-129981 (called "Reference B" hereinafter) that in the prior art field emitters as mentioned above, stability of emission is a problem.
It is known that the stability of emission depends upon a residual gas surrounding the field emitter and the temperature of a tip end of the field emitter. As a countermeasure for stabilization, the above referred References A and B propose to heat the filament by causing an electric current to flow through the filament, so as to control the temperature of the sharpened needle fixed to the filament for the purpose of stabilization
An operation in the above referred References A and B will be now explained with reference to FIG. 1C. Independently of a slight current caused to flow to the resistor B-D for the electron supplying, a current is supplied through a series-connected resistor formed of the resistor A-B and the resistor B-C so as to control the temperature by means of the resistance heating.
On the other hand, the Reference C provides a filament for the purpose of emitting a gas, independently of the field emitter used as an electron source of the electron gun, in one vacuum container, so that the residual gas is controlled by controlling the supplying of the power to the gas emitting filament, for the purpose of stabilizing the emission of the field emitter.
In the technology disclosed in the Reference C, since the gas emitting filament has to be located at a position which gives no influence to the function of the electron gun, the device inevitably becomes large in size, and also, an extra wiring for the gas emitting filament becomes necessary.
In the technology disclosed in the References A and B, on the other hand, the sharpened needle, which is the member for actually emitting the electrons, is not directly heated, but the filament, which is the member for supporting the sharpened needle, is heated by means of the resistance heating, so that an emission position is heated by a conduction heating or a radiation heating. In this case, it is sufficient if only the tip end of the needle which is the emission position, is heated, however, a large heat capacity including the filament is actually heated.
Furthermore, in the technology disclosed in the References A and B, not only the power supplying is increased for the heating, but also a gas is generated from the heated filament and its peripheral structure heated, so that the residual gas is increased in the proximity of the tip end of the field emitter. This results in a vicious spiral in which if the residual gas is increased, the temperature of the emitter tip end is required to be higher than that required before the residual gas increases.
In the field emitter shown in FIG. 1A, it is a general practice that the filament has the total length of 5 mm. The micro field emitter includes one having the structure shown in FIG. 1B, in which when the current is caused to flow through the resistor, the heating effect is obtained similarly to the resistance heating obtained by causing the current to flow through the filament. However, in the structure shown in FIG. 1B, the current flowing through the resistor is the current for supplying the electrons which are emitted from the tip end of the molybdenum projection, the current is increased dependently with the increase of the emission amount.
Namely, there occurs a cause-and-effect relation in which if the emission amount increases, the temperature of the resistor increases. In this characteristic relation, even if the amount of emission is controlled with a high frequency, a rising characteristics becomes dull. This is because of the phenomenon that when the emission rises up, namely, in the course in which the emission increases, it becomes gradually easy to emit the electrons with the increase of the temperature. Under a similar principle, the falling characteristics also becomes dull. The frequency characteristics deterioration caused with the temperature dependency, depends upon the heat capacity of the resistor.
In both the structure shown in FIG. 1A and the structure shown in FIG. 1B, since the sharpened needle and the molybdenum projection, which are the structural member for emitting the electrons, are not directly heated, it is necessary to heat the large heat capacity including the peripheral structure.